1. Technical Field
The present invention is directed to an apparatus and method for compensating for environmental effects on media. In particular, the present invention is directed to an apparatus and method for compensating for the effects of tape creep in magnetic tape media.
2. Description of Related Art
Magnetic tape recording has been utilized for many years to record voice and data information. For information storage and retrieval, magnetic tape has proven especially reliable, cost efficient and easy to use. In an effort to make magnetic tape even more useful and cost effective, there have been attempts to store more information per given width and length of tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, this increase in the number of data tracks results in those tracks being more densely packed onto the tape. As the data tracks are more closely spaced, precise positioning of the tape with respect to the tape head becomes more critical as errors may be more easily introduced into the reading or writing of data. The tape head positioning may be affected by variations in the tape or tape head, tape movement caused by air flow, temperature, humidity, tape shrinkage, and other factors, especially at the outside edges of the tape.
In order to increase data track accuracy, servo tracks have been employed to provide a reference point to maintain correct positioning of the tape with respect to the tape head. One or more servo tracks may be used depending upon the number of data tracks which are placed upon the tape. The sensed signal from the servo track is fed to a control system which moves the head and keeps the servo signal at nominal magnitude. The nominal signal occurs when the servo read gap is located in a certain position relative to the servo track.
Referring to FIG. 1, a one-half inch wide length of magnetic tape 11 may contain up to 288 or more data tracks on multiple data stripes 12. A thin film magnetic read head is shown in upper position 13 and lower position 14 to read data from data tracks 12. If a tape read head has sixteen elements and, with movement of the head to multiple positions, each element can read nine tracks, then that magnetic read head could read 144 tracks. In order to read more tracks, such as 288 in the desired configuration, two data bands 15 and 16 are utilized. The tape head is movable to nine tracking positions in each of upper position 13 and lower position 14. That is, with the tape head in position 13 it can read 144 tracks in data band 15 and in position 14 it can read 144 tracks in data band 16. With dual data bands 15 and 16 and multiple head positions within those bands, tape head positioning is critical.
In order to achieve accurate multiple head positions it may be desirable to include up to five or more servo stripes 17. Servo stripes 17 may utilize various patterns or frequency regions to allow precise tape to tape head positioning in multiple positions. This allows a data read head to more accurately read data from data stripes 12. Referring to FIG. 2, servo stripes 17 are shown in greater detail. As is disclosed in copending U.S. Pat. No. 6,023,385, entitled TAPE SERVO PATTERN WITH ENHANCED SYNCHRONIZATION PROPERTIES issued on Feb. 8, 2000, and hereby incorporated by reference, a first frequency signal 19 is written across the width of a frame 18 in each servo stripe 17. As is known in the art, a measurably different frequency signal such as an erase frequency is written over first frequency signal 19 in a predetermined pattern such as the checker board patterns in regions 21 and 22. The horizontal sides of twelve rectangles 20 and 23 in each stripe 17 are substantially parallel to the direction of movement of tape length 11. The six rectangles (12 sides) in each region 21 and 22 define five horizontal interfaces (servo tracks) 24 between frequency signal 19. Rectangles 20 and 23 as the outside interfaces 25 along the top and bottom of each stripe 17 are ignored. In the preferred embodiment rectangles 20 are shown on the left side of areas 21 and 22 and rectangles 23 are shown on the right portion of areas 21 and 22. A servo read element 26 in a tape read head is precisely aligned along interface 24 to read the signal frequency along interfaces 24. That is, dotted line representing interface 24 along the horizontal sides of rectangles 20. 23 passes through the center of servo read element 26. If the servo pattern on the tape moves right to left, then servo read element 26 will alternate between reading frequency 19 across the full width of servo read element 26 and an erase frequency from rectangles 20. 23 across the other half of the width of servo read element 26. Thus, if tape 11 moves as shown in FIG. 2, servo read element 26 will first sense rectangle 20 above track 24 and then sense rectangle 23 below track 24 in each of regions 21 and 22.
As is known in the art, the servo control system in a tape drive determines the position error signal (PES) by using the ratio of the difference between the signal amplitude sensed during the first (left) half of patterns 21 or 22 and the signal amplitude sensed during the second (right) half of patterns 21 or 22 divided by the sum of the signal amplitude sensed during the first half of patterns 21 or 22 and the signal amplitude sensed during the second half of patterns 21 or 22 to stay on track. For a head position precisely on track in checkerboard pattern areas 21 or 22 shown in FIG. 2 the ratio will be zero because the signal amplitude during each half of the pattern will be the same. If servo read element 26 is above track 24, the position error signal will be non-zero because less of the erasure area is read and thus, the amplitude of the signal is not reduced to zero. In response, the track servo will move the head (including servo read element 26) down until the ratio is zero and servo read element 26 is precisely on track 24. Conversely, if servo read element 26 is below track 24, the polarity of the position error signal will be negative because more of rectangle 23 below track 24 and less of rectangle 20 above track 24 will be read. In response, the track servo will move the head (including servo read element 26) up until the ratio is zero and servo read element 26 is precisely on track 24. In this way the tape controller can determine the position of the tape 11 with respect to the servo read element 26 and move the tape head to keep the head servo read element 26 aligned with the servo track along line 24. This alignment ensures precise reading of a data track in data stripes 12 by the data read head (not shown).
Over the life of a magnetic media, such as magnetic tape, the configuration of the magnetic media may become warped or otherwise changed from the original configuration of the magnetic media. For example, due to stresses applied to a magnetic tape, the tape width may begin to migrate, i.e. the tape may become bowed-in at the edges or bowed-outward. This phenomenon is known as tape creep. Because of tape creep, the servo reader elements of a read head may not be properly positioned relative to the servo tracks on the magnetic media. Hence, the data reader elements will not be properly positioned relative to the data tracks on the magnetic media. This may lead to errors in reading information from the data tracks.
Furthermore, the position of conventional servo reader elements may not be able to be adapted to compensate for the tape creep phenomenon. This is because known read head adjustment methods only reposition the position of the read head in a perpendicular direction to the magnetic tape, i.e. with reference to FIG. 1, in a vertical direction relative to the magnetic tape. These known adjustment methods are directed to compensating for a shift of the magnetic tape relative to the read head. These adjustment methods do not take into account the problems associated with tape creep.
FIG. 3 is an exemplary block diagram of a redundant reader apparatus illustrating the tape creep phenomenon. In FIG. 3, the original track positions are shown as dashed lines while the current track positions, due to tape creep, are shown as solid lines. The position of the outer tracks 340 and 360 has migrated relative to the center track 350. As a result, the position of the servo readers 310, 320 and 330, which are properly positioned about the original track positions 345, 355 and 365 on the magnetic media 300, is out of alignment with respect to the current position of the servo tracks. Therefore, the mis-alignment of the data readers 370 to the data tracks worsens as the distance between a given data reader and an associated servo reader 320 increases. Furthermore, the outer servo readers 310 and 330 may be at a position that is either above or below the servo track so that correct reading of the servo track cannot be accomplished.
In general, as long as the average error bias obtained from the servo readers is within a reasonable range, the data tracks may still be able to be read taking into account the error bias. However, in the case of tape creep, the error bias may be so large that the outer servo and data tracks may not be able to be read or written.
If a conventional adjustment method were used to attempt to compensate for the tape creep shown in FIG. 3, the result would be that the position of some of the servo readers may be properly positioned, but others would not be. For example, servo reader 320 may be able to be properly positioned, but the position of servo readers 310 and 330 would still be incorrect. This is due to the fact that, in the known adjustment methods and apparatus, all of the servo readers 310-330 of a read head must be moved the same amount to compensate for differences in position with regard to the servo tracks. This is because the position of the servo readers 310-330 are fixed with respect to the data readers and with each other.
One method of compensating for tape creep while still using the known methods of read head adjustment is to use a more durable medium. For example, Aramid, which is more durable than conventional magnetic tape medium, may be used as the magnetic tape medium. However, Aramid is much more expensive than conventional magnetic tape medium. Thus, the cost involved in using this solution to the tape creep problem may be prohibitive. Therefore, it would be advantageous to have an apparatus, system and method for compensating for changes in the configuration of magnetic media due to environmental effects.
An apparatus and method for compensating for environmental effects on media. The apparatus includes an array of servo readers that are positioned at a non-zero azimuth angle relative to a medium. The position of the array of servo readers is dynamically updateable in both a vertical direction and an azimuth angle direction based on a combination of an average position error signal of both an upper portion of the servo readers and a lower portion of the servo readers. Based on the average position error signal and a reference signal, a controller generates control signals that are output to one or more actuators. The actuators then adjust the position of the array of servo readers based on the control signals received.